1. Field of the Invention
The present invention relates to an antenna for use in a wireless communication device that utilizes electromagnetic wave such as microwave or millimeter wave. The present invention can be used particularly effectively in a wireless LAN (local area network) used in office and homes and mobile communications terminals such as cell phones.
2. Description of the Related Art
Conventional RF circuits for use in wireless communication devices that utilize microwave to millimeter wave frequency bands include circuits that use a coaxial line or a waveguide tube and circuits that use a planar substrate. Generally speaking, circuits using a coaxial line or a waveguide tube have a low loss but often make a thick, heavyweight and lengthy system. On the other hand, a microstrip circuit, a coplanar circuit and other circuits fabricated on a planar substrate tends to have an increased transmission loss but are flat, small-sized and lightweight. In addition, those circuits also have beneficial features that they can be formed easily as printed circuits on a dielectric substrate and that various surface-mount semiconductor devices can be used thereon. That is why an antenna taking advantage of these features is often used as a wireless circuit in a mobile communications terminal station for a cell phone or a wireless LAN.
There is often a radio wave obstruction such as something shielding or reflecting the radio wave between a mobile communications terminal station and a base station. Besides, the radio wave propagation environment frequently changes in a complicated manner due to the shift of the location of such a radio wave obstruction or mobile communications terminal station. On top of that, the mobile communications terminal station should be as small-sized and lightweight as possible, and therefore, can use only a limited quantity of power. For that reason, to maintain wireless communication as long as possible, the power dissipation is preferably minimized.
To maintain the wireless communications link at an appropriate level under such an environment, the antenna radiation properties (e.g., the gain and the directivity) of the mobile communications terminal station are preferably adaptively changeable according to the situation. More specifically, the directivity of the antenna at the terminal station is preferably changed dynamically into a direction in which connection can be established appropriately with the antenna at the base station. This requirement should be satisfied more fully in making communications over the high frequency band (e.g., millimeter wave), in particular.
Hereinafter, a microstrip antenna, which is a typical conventional planar antenna, will be described with reference to FIG. 17. A typical conventional microstrip antenna is described in Japanese Patent Application Laid-Open Publication No. 5-343915, for example.
FIG. 17 schematically illustrates the microstrip antenna disclosed in Japanese Patent Application Laid-Open Publication No. 5-343915. The antenna shown in FIG. 17 includes a dielectric layer 701, a driven element 702 provided on the upper surface of the dielectric layer 701, a grounded conductor 703 provided on the lower surface of the dielectric layer 701, a non-driven element 704 provided so as to face the driven element 702, a dielectric substrate 705 located under the grounded conductor 703, and a microstrip line 706 located on the lower surface of the dielectric substrate 705. A slot 707 is defined in the grounded conductor 703 and is located between the driven element 702 and the microstrip line 706. The driven element 702 and non-driven element 704 are square in FIG. 17 but may also have a circular shape.
As can be seen from FIG. 17, the driven element 702 and the microstrip line 706 are arranged so as to sandwich the grounded conductor 703 between them, and the slot 707 is located under the center portion of the driven element 702. Thus, the microwave that has propagated through the microstrip line 706 is coupled to the electromagnetic field in the antenna by way of the slot 707, thereby exciting a fundamental-mode electromagnetic field in the antenna. FIG. 18 shows a radiation pattern in a situation where such a mode has been excited.
In maintaining wireless communication either through a mobile communications terminal station or in a room where a number of persons go back and forth frequently, the radio wave propagation environment changes successively due to shielding or reflection as described above. For that reason, to keep up a good communication link, the antenna properties are preferably controllable adaptively.
In the conventional antenna shown in FIG. 17, however, various properties thereof such as the directivity, gain and efficiency are determined by its fixed antenna shape. That is why it is difficult to change those various antenna properties dynamically in response to any change in radio wave propagation environment.
Also, even if the antenna properties do not have to be changed dynamically, the properties of the antenna being designed are still preferably assessed while changing the antenna shape such that the best antenna properties can be adopted according to various environments.
Japanese Patent Application Laid-Open Publication No. 62-196903 discloses a planar antenna in which a number of microstrip line conductors are arranged over the entire surface. In such a planar antenna, the distance between the surface on which the array of microstrip line conductors is provided and the grounded conductor is changed according to the situation. However, in a planar antenna with such a structure, that distance is changed by shifting the grounded conductor entirely. Accordingly, there are just a few parameters that affect the antenna properties and the variation range of the antenna properties is too narrow.